Starboard engine occasionally loses thrust at higher rpms when cruising

Gucci

Gucci

Member
Dec 25, 2017
61
Boat Info
330 Sundancer (1995)
Engines
Twin Mercury 350
Starboard engine occasionally loses thrust at higher rpms, engine sound becomes more metallic, what could be the reason? When I do the same and push the throttle when in neutral, it does not happen.
 
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Normally, when you push the throttle forward. the boat should go faster, but the it does not it stalls.
 
What engines, what transmissions? Is it a mechanical or electrical shift? I’ve had similar issues, but need more info
 
Mercury, mechanical shift not sure about what transmission it is. Sometimes engine will quit on its own on idle.
 
Do you know which carbs you have? The good news is that nearly every carb you might have probably has several rebuild videos on youtube.

A bad accelerator pump will elicit the symptoms you're seeing, but so could gunk/clogs. I have Mercarbs, and the design of the accel pump is a head-scratcher to me - it looks like something a 2nd-grader thought up.

Once you know the carb is healthy, you might want to make sure the plugs are ok - a carburetor with problems can quickly trash even a good set of plugs.
 
There are a ton of very smart people on the forum, but you limit their sharing of knowledge because you are limiting the details one needs to get you the answer or direction you need.
What year and style is your boat?
What merc engine, 454?
Carb or injected
What trany? The number is on the tag on top of the trany.
Describe in detail the rpm range the engine cuts out. Does stop or stay running when you pull the throttle back?

A doctor cant prescribe meds unless you give him details of the symptoms. Sorry same with boats.

Mark
 
Based on your limited description via metallic sound. I'll guess the clutch plate is worn and smoothed out. Hence it cant hold the torque from the engine and its slipping.
If so, the main cause for a slipping worn clutch plate is throttling up to fast. Like zero to full like a rocket. The plates are tough but that force wears them out super fast. Not saying you do that. But that's what can happen. Another way those wear faster is the idle rpm is to high so when engaging the clutch the friction takes to long to lock in place. Over and over that wears the plate.

If that is the reason for your issue. The trany will need to come out to replace the plate. Approx a 15 hour job.
 
LG111 said:
There are a ton of very smart people on the forum, but you limit their sharing of knowledge because you are limiting the details one needs to get you the answer or direction you need.
What year and style is your boat?
What merc engine, 454?
Carb or injected
What trany? The number is on the tag on top of the trany.
Describe in detail the rpm range the engine cuts out. Does stop or stay running when you pull the throttle back?

A doctor cant prescribe meds unless you give him details of the symptoms. Sorry same with boats.

Mark
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I am new to boating, do not know yet enough about engines/mechanics, thanks for the reply. Will provide more info when I get to the boat.
 
LG111 said:
Based on your limited description via metallic sound. I'll guess the clutch plate is worn and smoothed out. Hence it cant hold the torque from the engine and its slipping.
If so, the main cause for a slipping worn clutch plate is throttling up to fast. Like zero to full like a rocket. The plates are tough but that force wears them out super fast. Not saying you do that. But that's what can happen. Another way those wear faster is the idle rpm is to high so when engaging the clutch the friction takes to long to lock in place. Over and over that wears the plate.

If that is the reason for your issue. The trany will need to come out to replace the plate. Approx a 15 hour job.
Click to expand...
This might actually be the issue, it does not lose thrust on a consistent basis, will get back with more info after taking it to the mechanic, thanks for your reply.
 
Gucci said:
This might actually be the issue, it does not lose thrust on a consistent basis, will get back with more info after taking it to the mechanic, thanks for your reply.
Click to expand...
Good luck on the issue. Welcome to boating! Get to know your boat inside and out. Learn all the systems and wiring. Get very comfortable with the engines and what each part does. Study study study. Take things apart and learn to put them back together. You'll appreciate your future self someday when she breaks down miles from port and you'll be the one to fix her! That's boating at its finest!
 
So, it turns out, it is not the transmission, not the plate, not the carburetor, not the computer, but the engine 8.1 mercruiser, is still doing the same thing (losing thrust) at 3,500 rpm less than that it works fine. We replaced the coils, exhaust pipes, checked heat exchanger (disc was cracked, replaced that) checked exhaust risers. One thing though the engine stays cool does not heat up (after the impellers were replaced) to normal temperature between 130-170. At this point it is still a puzzle, we are open to any suggestions at the moment.
 
dtfeld said:
What engines, what transmissions? Is it a mechanical or electrical shift? I’ve had similar issues, but need more info
Click to expand...
8.1 mercruiser, mechanical, automatic transmission
 
Have the same motors (496 mags) and been chasing the same type of issue (all last season). Would be cruising at 3400 and the rpms would drop 2500-3000 briefly (just long enough for you to feel it) and return to the 3400 withing a couple of seconds at most. It is always the same amount of drop but would do it more and more often the longer we were out. End of last season had transom assemblies done and the mechanic also helped me chase this ghost around when we went out for test run after transom assemblies done. Had the computer hooked up and was getting some voltage variations to different sensors. After some discussion, he asked the age of my batteries. They are more than 4 seasons old and if left of charge for longer than a few days, will not start the motor. After looking at them, they are bowed out on the sides and his first instinct was to replace the batteries (since they are due anyway). So . . .Plan is to replace batteries for that motor this spring and see what happens. He explained that the Mercury computers are very voltage/amperage sensitive and just 1 poor battery can cause really weird stuff, just as this.
 
Maybe so, but the bulging sides were not noticeably present until mid season last year. Our boat is also kept at our house in a boat lift and in a boathouse. The cabin and engine room are both heated to 50 degrees and monitored which sends notification if temps fall below 40 degrees. I do know freezing may cause this, but it didn't happen on our watch. We bought our 340 4 seasons ago. The Port batteries (2) were replaced when purchased. The Stbd batteries (3), one is bow thruster, were said to be relatively new. There was no documentation avail as to this. So we really don't know their exact age. They are also sealed AGMs, so topping off not an option. Going to replace cause they need it anyway. If ghost goes away, that is a bonus. If not, may need to hire a ghost buster lol..
 
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